Adaptive Equalizer for a Coded Speech Signal

1. A speech communication system, comprising: a speech decoder that receives a set of coded parameters representative of the desired signal characteristics without explicit quantization and transmission of information about an equalizer response and inputting quantized, and uses the set of coded parameters and the inputting quantized spectral coefficients to generate reconstructed speech, said speech decoder comprising an equalizer that computes equalizer response including a matching set of speech coder parameters from the reconstructed speech that match speech coder parameters that were quantized by a speech encoder before the speech encoder transmitted the set of coded parameters representative of the desired signal characteristics to the speech decoder, undoes the set of characteristics corresponding to the computed set of speech coder parameters, and imposes the set of characteristics corresponding to the coded set of speech coder parameters, thereby producing equalized reconstructed speech.

2. The speech communication system of claim 1 , wherein the set of coded parameters representative of the desired signal characteristics is the set of spectral coefficients.

3. The speech communication system of claim 2 , wherein the spectral coefficients are linear prediction (LP) coefficients for a short-term filter.

4. The speech communication system according to claim 1 , wherein the speech decoder further comprising: a demultiplexer that demultiplexes a received coded bitstream to recover therefrom quantized spectral (LP) coefficients and excitation parameters corresponding to a frame in a sequence of speech frames, the excitation parameters comprising a codevector index, a scale factor, long term predictor filter coefficients and a delay value; a codebook that stores a plurality of codebook codevectors with each of the plurality of codebook codevectors associated with an index for generating a codebook codevector in response to the recovered codevector index; a long-term predictor filter that processes the codebook codevector using the long term predictor filter coefficients and the delay value recovered for the frame in the sequence of speech frames to generate a combined excitation signal; and an LP synthesis filter that processes the combined excitation signal using the recovered quantized spectral coefficients to generate a reconstructed speech signal corresponding to the frame in the sequence of speech frames.

5. The speech communication system according to claim 4 , wherein the excitation parameters further comprise a scale factor, and wherein the speech decoder further comprises: a gain controller, coupled to said codebook and responsive to the recovered scale factor, for generating a scaled codebook codevector; and said long-term predictor filter processes the scaled codebook codevector using the long term predictor filter coefficients and the delay value recovered for the frame in the sequence of speech frames to generate a combined excitation signal.

6. The speech communication system according to claim 1 , wherein said equalizer computes from the reconstructed speech signal and quantized spectral coefficients recovered from a received coded bitstream an equalizer response, the equalizer response being used to generate the equalized reconstructed speech.

7. The speech communication system according to claim 6 , wherein said equalizer computes the equalizer response by applying an LP analysis window to the reconstructed speech signal to generate a windowed reconstructed speech signal, analyzing the windowed reconstructed speech signal using LP analysis to derive therefrom spectral (LP) coefficients, generating an impulse response using a zero-state zero filter response defined by the derived spectral (LP) coefficients, filtering the impulse response using a zero-state pole filter response defined by the recovered quantized spectral coefficients to generate an initial equalizer impulse response, transforming the initial equalizer impulse response using a Fast Fourier Transform into a frequency domain signal, calculating the magnitude spectrum of the frequency domain signal, using the magnitude spectrum as the equalizer magnitude response, setting the equalizer phase response to zero to generate an intermediate equalizer frequency response, and outputting the intermediate equalizer frequency response.

8. The speech communication system according to claim 7 , wherein said equalizer further computes the equalizer response by transforming the intermediate equalizer frequency response into an intermediate equalizer impulse response using an Inverse Fast Fourier Transform, and outputting the intermediate equalizer impulse response.

9. The speech communication system according to claim 8 , wherein a reconstructed speech signal is equalized by applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, convolving the windowed reconstructed speech frame using the intermediate equalizer impulse response to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

10. The speech communication system according to claim 8 , wherein said equalizer further computes the equalizer response by windowing the intermediate equalizer impulse response using a symmetric window to generate an equalizer impulse response, and outputting the equalizer impulse response.

11. The speech communication system according to claim 10 , wherein a reconstructed speech signal is equalized by applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, convolving the windowed reconstructed speech frame using the equalizer impulse response to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

12. The speech communication system according to claim 10 wherein said equalizer further computes the equalizer response by transforming the equalizer impulse response using a Fast Fourier Transform into an equalizer frequency response, and outputting the equalizer frequency response.

13. The speech communication system according to claim 12 , wherein a reconstructed speech signal is equalized by applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, zero padding the windowed reconstructed speech frame to generate a zero-padded windowed reconstructed speech frame, transforming the zero-padded windowed reconstructed speech frame using a Fast Fourier Transform to generate complex spectral coefficients, modifying the complex spectral coefficients by applying the equalizer frequency response to generate modified complex spectral coefficients, transforming the modified complex spectral coefficients using an Inverse Fast Fourier Transform to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

14. The speech communication system according to claim 6 , wherein a reconstructed speech signal is equalized by applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, zero padding the windowed reconstructed speech frame to generate a zero-padded windowed reconstructed speech frame, transforming the zero-padded windowed reconstructed speech frame using a Fast Fourier Transform to generate complex spectral coefficients, modifying the complex spectral coefficients by applying the intermediate equalizer frequency response to generate modified complex spectral coefficients, transforming the modified complex spectral coefficients using an Inverse Fast Fourier Transform to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

15. A method by which an equalizer equalizes a reconstructed speech signal without explicit quantization and transmission of information about an equalizer response, the method comprising the steps of: inputting the reconstructed speech signal inputting quantized spectral coefficients, computing equalizer response including a set of speech coder parameters from the reconstructed speech that match speech coder parameters that were quantized by a speech encoder before the speech encoder transmitted the set of coded parameters representative of the desired signal characteristics to the speech decoder, undoing the set of characteristics corresponding to the computed set of speech coder parameters, and imposing the set of characteristics corresponding to the coded set of speech coder parameters, thereby generating equalized reconstructed speech from the reconstructed speech signal and the quantized spectral coefficients.

16. The method according to claim 15 , further comprising the steps of: applying an LP analysis window to the reconstructed speech signal to generate a windowed reconstructed speech signal, analyzing the windowed reconstructed speech signal using LP analysis to derive therefrom spectral (LP) coefficients, generating an impulse response using a zero-state zero filter response defined by the derived spectral (LP) coefficients, filtering the impulse response using a zero-state pole filter response defined by the recovered quantized spectral coefficients to generate an initial equalizer impulse response, transforming the initial equalizer impulse response using a Fast Fourier Transform into a frequency domain signal, calculating the magnitude spectrum of the frequency domain signal, using the magnitude spectrum as the equalizer magnitude response, setting the equalizer phase response to zero to generate an intermediate equalizer frequency response, and outputting the intermediate equalizer frequency response.

17. The method according to claim 16 , further comprising: transforming the intermediate equalizer frequency response into an intermediate equalizer impulse response using an Inverse Fast Fourier Transform, and outputting the intermediate equalizer impulse response.

18. The method according to claim 17 , further comprising: applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, convolving the windowed reconstructed speech frame using the intermediate equalizer impulse response to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

19. The method according to claim 17 , further comprising: windowing the intermediate equalizer impulse response using a symmetric window to generate an equalizer impulse response, and outputting the equalizer impulse response.

20. The method according to claim 19 , further comprising: applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, convolving the windowed reconstructed speech frame using the equalizer impulse response to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

21. The method according to claim 19 , further comprising: transforming the equalizer impulse response using a Fast Fourier Transform into an equalizer frequency response, and outputting the equalizer frequency response.

22. The method according to claim 21 , further comprising: applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, zero padding the windowed reconstructed speech frame to generate a zero-padded windowed reconstructed speech frame, transforming the zero-padded windowed reconstructed speech frame using a Fast Fourier Transform to generate complex spectral coefficients, modifying the complex spectral coefficients by applying the equalizer frequency response to generate modified complex spectral coefficients, transforming the modified complex spectral coefficients using an Inverse Fast Fourier Transform to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.

23. The method according to claim 15 , further comprising: applying a synthesis window to the reconstructed speech signal to generate a windowed reconstructed speech frame in a sequence of reconstructed speech frames, zero padding the windowed reconstructed speech frame to generate a zero-padded windowed reconstructed speech frame, transforming the zero-padded windowed reconstructed speech frame using a Fast Fourier Transform to generate complex spectral coefficients, modifying the complex spectral coefficients by applying the intermediate equalizer frequency response to generate modified complex spectral coefficients, transforming the modified complex spectral coefficients using an Inverse Fast Fourier Transform to generate a modified windowed reconstructed speech frame, generating the equalized reconstructed speech signal using an overlap/adder on adjacent modified windowed reconstructed speech frames, and outputting the equalized reconstructed speech signal.